1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor comprising a photosensitive layer having a layered structure comprising a charge generating layer containing a titanyl phthalocyanine composition as a charge generating substance and a charge transporting layer containing a bisamine compound as a charge transporting substance, and more particularly it relates to an electrophotographic photoreceptor that is applied to a digital image forming apparatus, in which a latent image is formed on an electrophotographic photoreceptor by high density exposure of 1,200 dpi or more, and the latent image is visualized by a reverse development method using a toner having an average particle diameter of 6 .mu.m or less.
2. Description of the Related Art
An electrophotographic technique is employed in recent years not only in the field of duplicators, but also in the fields of various printers and facsimile machines owing to its excellent instantaneousness and provision of an image having high quality and high preservability. The electrophotographic process generally comprises a step of uniformly charging an electrophotographic photoreceptor (hereinafter sometimes simply referred to as "photoreceptor"), a step of forming a latent image by exposure, a step of developing the latent image with a toner, and a step of transferring and fixing of the toner image to a receiving material. In some cases, the toner image is transferred to an intermediate receiving material, and the transferred to then receiving material.
As the photoreceptor, photoreceptors comprising an organic series material, which causes no environmental pollution and can be conveniently formed into a photosensitive layer, are developed, as well as photoreceptors comprising an inorganic series material, such as selenium, an arsenic-selenium alloy, cadmium sulfide and zinc oxide. Furthermore, in a photoreceptor of a function-separated type having a layered structure of a charge generating layer and a charge transporting layer, such advantages can be obtained that high sensitivity is obtained, materials can be selected from wide ranges, high safety can be obtained, and it can be produced at a low cost by a coating method of high productivity.
In recent years, on the other hand, digitization of image formation quickly proceeds in order to obtain an image of higher quality or to memorize and arbitrarily edit an input image. According to the trend, the digital image formation spreads to the field of general duplicators in that the analog image formation has been the mainstream, which has been limited to a laser printer and an LED (light emission diode) printer used as an output apparatus for a word processor and a computer, and a part of a color laser duplicator.
In a photoreceptor compliant to the digital image formation, a laser light source and an LED are used as recording means for recording image information on the photoreceptor. In the recording means, in particular, a near infrared light source of 780 nm and a red light source of 650 nm are frequently used. Therefore, high sensitivity to light of these wavelengths is demanded, and a crystalline phthalocyanine composition, particularly a crystalline titanyl phthalocyanine composition of high sensitivity, is being studied as a material realizing such a photoreceptor of high sensitivity. The crystalline titanyl phthalocyanine composition includes various crystalline forms, in which the size and form of the crystalline lattice are determined by agreement and disagreement of diffraction peaks in an X-ray diffraction spectrum, and the molecular arrangement in the crystalline lattice is determined by the relative intensities of the diffraction peaks. In the photoreceptor using the crystalline titanyl phthalocyanine, the charging property, the dark attenuation property and the sensitivity are greatly changed by the size and form of the crystalline lattice and the molecular arrangement in the crystalline lattice.
As described in the foregoing, the digital image formation employs a laser light source and an LED as the recording means for recording image information on the photoreceptor. In this case, because an image is expressed by an array and an arrangement of minute dots called a pixel, a formation technique of minute spots owing to high resolution of an optical system is required, and a recording density of 1,200 dpi or more is realized by the optical system. Accordingly, a photoreceptor compliant to the high density recording of 1,200 dpi or more is demanded.
Japanese Examined Patent Publication JP-B2 2696400 discloses an image forming technique, in which digital image exposure is conducted at a recording density of 600 dpi or more, and a toner having a weight average particle diameter of 8 .mu.m or less is used. In the digital image formation of a high recording density of 1,200 dpi or more, however, it is difficult to faithfully reproduce a static latent image on a photoreceptor only by determining the weight average particle diameter of the toner as in the patent publication. Furthermore, it is necessary to design to prevent deterioration in recording density of the photoreceptor.
According to the demand of high sensitivity and long service life, a photosensitive layer having a large thickness is studied. For example, such a photosensitive layer is disclosed in Japanese Unexamined Patent Publications JP-A 3-11353 (1991), JP-A 3-63653 (1991), JP-A 3-87749 (1991), JP-A 3-56966 (1991), JP-A 6-301224 (1994), JP-A 7-244388 (1995) and JP-A 7-261415 (1995).
The conventional crystalline titanyl phthalocyanine composition of used as a charge generating substance has problems in the stability of the crystalline system and the stability of the dispersion of the crystalline substance. Furthermore, in the case where a photoreceptor having the conventional photosensitive layer is applied to an image forming apparatus of a reverse development method, there are problems in the initial stability of the charge potential, particularly the stability of the first charge potential after dark adaptation, and charge ability, i.e., the capability of charge maintenance in an initial stage and after the life. There is also a problem in that the potential characteristics are changed by the change of the environment, particularly change of the temperature, to cause formation of minute image defects. Therefore, the photoreceptors of the conventional technique cannot fully satisfy the demand of high sensitivity, high image quality and high stability.
Furthermore, high resolution of the photoreceptor itself is demanded due to proceed of high image quality, and therefore a photoreceptor of high sensitivity is required that faithfully reproduce a charge latent image with high density recording of 1,200 dpi or more, for example, high density recording of from 1,500 to 2,400 dpi. While the thickness of a photosensitive layer of a photoreceptor used for a recording density of 600 dpi or less is from 20 to 35 .mu.m, the thickness of the photosensitive layer is set with consideration of the sensitivity required in the photoreceptor and printing durability (service life). Thus, the reproducibility of a static latent image on the photoreceptor is not considered since such does not cause any particular problem. However, when the thickness of a photosensitive layer of a photoreceptor used at a recording density of 1,200 dpi is 20 .mu.m or more, it is difficult to faithfully reproduce a charge latent image.
When the thickness of the photosensitive layer of the photoreceptor used for high density recording is 20 .mu.m or more, the charge is diffused depending on the transportation distance of the charge within the photosensitive layer to deteriorate the resolution. For example, according to simulation, when the thickness of the photosensitive layer is 30 .mu.m, the deterioration of the charge latent image due to diffusion of charge spreads to about 25 .mu.m. In order to prevent deterioration in resolution on forming a charge latent image in a photoreceptor requiring high resolution, the surface charge density is increased, or the thickness of the photosensitive layer is decreased to such a level that the deterioration due to the diffusion of charge does not cause problem. When the thickness of the photosensitive layer is decreased, however, a problem is caused in that the electric field intensity applied to the photosensitive layer is increased to lower the voltage resistance. The substantive sensitivity is also decreased due to increase of the electric capacitance. The decrease in voltage resistance brings about formation of minute image defects in the reverse development. The decrease in substantive sensitivity brings about decrease in contrast, and therefore in order to ensure the sufficient image density, the surface potential has to be further increased, and the electric power of the light source has to be increased.
Japanese Examined Patent Publication JP-B2 5-55860 (1993) discloses a lamination type electrophotographic photoreceptor comprising a charge generating layer formed with a coating composition comprising a titanyl phthalocyanine compound dispersed in a binder resin and a charge transporting layer containing a hydrazone compound. However, it cannot satisfy all the requirements, i.e., high sensitivity, high image quality, high stability and high resolution. As a photoreceptor satisfying all the requirements, an a-Si series photoreceptor achieving a sensitivity of 0.1 .mu.J/cm.sup.2 can be exemplified. However, such a photoreceptor involves problems in environment and layer forming property. Thus, an organic series photoreceptor is demanded, in which no environmental pollution is caused, the formation of layers can be conducted conveniently, i.e., the production thereof is convenient, and all the requirement are satisfied. Furthermore, in particular, a coating composition that can be suitably used for forming a charge generating layer of the organic series photoreceptor is also demanded.